Acknowledgments 176

References 176


A central problem in motor control is to understand how the brain coordinates the trajectories of simultaneously moving and coupled body segments, so as to achieve a common goal. “Both the difficulty and fascination of this problem lie in the apparent conflict between two fundamental properties of the motor system: the ability to accomplish high-level goals reliably and repeatedly, versus variability on the level of movement details.”1 We focus here primarily on a striking example of multisegment motor control, namely the use of coordinated eye and head movements to displace rapidly the visual axis (called a gaze shift). The important constraints on end-point accuracy imposed by the need to foveate a target within the context of potentially highly variable eye and head trajectories2 exemplify the challenge to the neural gaze controller. Particularly striking is that perturbations of a gaze trajectory are compensated such that movement performance is maintained. While research on the limb and eye/gaze motor systems has evolved largely independently, current theoretical models in both fields have emphasized feedback control to address coordination and accuracy. In this chapter, we consider the feedback controller approach for eye-head gaze saccades made by head-unrestrained subjects.